A risk situation of organic pollutants accumulation in the sediments of the dams from Cascade “Middle Iskar” (NW Bulgaria) is simulated in the present study. The aim is risk modelling of organic pollution in lake sediments and on base of the investigated key chemical and enzymological parameters to risk assessment. A model process in lab scale with addition of whey as a source of organic pollutants was simulated. Two situations near to real conditions were studied: 1) at a decreased concentration of organic pollutants (chemical oxygen demand was 0.265 gO2.kg-1) in the case of discharge of sediments downstream of the river (control situation); 2) at an accumulation of organic pollutants (chemical oxygen demand was 1.463 gO2.kg-1) in the sediments (risk event). The dynamics of chemical oxygen demand (COD), concentration of proteins and lactose as well as of the key enzymological indicators (phosphatase activity index—PAI, anaerobic dehydrogenase activity, protease and ?-galactosidase activities) were investigated. The data from Risk Assessment showed that there is a hazard from higher protein concentration at the end of the process in comparison with the control situation. The risk about COD and lactose concentration during the risk modelling was acceptable. The obtained results showed that the PAI can be used as an indicator for early warning of the organic pollution with protein nature.
 Todorova, Y., Lincheva, S. and Topalova, Y. (2014) Risk Management Scenario for Multiple Heavy Metal Contamination at River Sediments in the Middle Iskar Cascade. Bulgarian Journal of Agricultural Science, 20, 100-104.
 Bulgarian Ministry of Environment and Water (2010) Danube River Basin Management Plan.
 Bitton, G. (2005) Wastewater Microbiology. 3rd Edition, John Wiley & Sons, Inc., Hoboken.
 Danalewich, J.R., Papagiannis, T.G., Belyea, R.L., Tumbleson, M.E. and Raskin, L. (1998) Characterization of Dairy Waste Streams, Current Treatment Practices, and Potential for Biological Nutrient Removal. Water Research, 32, 3555-3568.
 Ganoulis, J. (2009) Risk Analysis of Water Pollution. 2nd Edition, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
 Lincheva, S., Todorova, Y. and Topalova, Y. (2014) Long-Term Assessment of Self-Purification Potential of Technologically Managed Ecosystem: The Middle Iskar Cascade. Biotechnology and Biotechnological Equipment, 28, 455- 462.
 Bobba, A.G., Singh, V.P. and Bengtsson, L. (2000) Application of Environmental Models to Different Hydrological Systems. Ecological Modelling, 125, 15-49.
 Bredeweg, B., Salles, P., Bouwer, A., Liem, J., Nuttle, T., Cioaca, E., Nakova, E., Noble, R., Caldas, A.L.R., Uzunov, Y., Varadinova, E. and Zitek, A. (2008) Towards a Structured Approach to Building Qualitative Reasoning Models and Simulations. Ecological Informatics, 3, 1-12.
 Liu, H., Li, L., Yin, C. and Shan, B. (2008) Fraction Distribution and Risk Assessment of Heavy Metals in Sediments of Moshui Lake. Journal of Environmental Science, 20, 390-397.
 Yang, J., Chen, L., Liu, L.Z., Shi, W.L. and Meng, X.-Z. (2014) Comprehensive Risk Assessment of Heavy Metals in Lake Sediment from Public Parks in Shanghai. Ecotoxicology and Environmental Safety, 102, 129-135.
 Das, B., Nordin, R. and Mazumder, A. (2008) An Alternative Approach to Reconstructing Organic Matter Accumulation with Contrasting Watershed Disturbance Histories from Lake Sediments. Environmental Pollution, 155, 117-124.
 Miller, G.L. (1959) Use of Dinitrosalicylic Acid Reagent for Determination of Reducing Sugar. Analytical Chemistry, 31, 426-428.
 Ladd, J.N. and Butler, J.H.A. (1972) Short-Term Assay of Soil Proteolytic Enzyme Activities Using Proteins and Dipeptide Derivates as Substrates. Soil Biology and Biochemistry, 4, 19-39.
 Trevors, J., Mayfield, C. and Inniss, W. (1982) Measurement of Electron Transport System (ETS) Activity in Soil. Microbial Ecology, 8, 163-168.
 EC Directive (2000) Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 Establishing a Framework for Community Action in the Field of Water Policy. Official Journal of the European Communities, L327, 1-72.